A territory controlled and administered by the British army should be considered subject to British law. That is the very sensible view of the Law Lords, who ruled last week that the Human Rights Act extends to civilians in the custody of British soldiers in Iraq. They were considering the case of Baha Musa, a hotel worker who was detained by the Queen's Lancashire Regiment in Basra in 2003. He was hooded, kept in a stress position and then beaten to death.

The fact that Musa's human rights were contravened in the course of his brutal ordeal is so patently obvious it is a wonder that the supreme court of the land had to be asked for its opinion on the matter.

But the army was in no hurry to take responsibility for Musa's death. An internal investigation, which was, according to a military judge, obstructed by a 'closing of ranks', resulted in a court martial that acquitted six soldiers. One pleaded guilty to inhumane treatment of prisoners. The killers have not been found. So Musa's father has had to seek justice from a higher authority.

Musa was not killed in the heat of battle, a distinction the Law Lords made when deciding what constituted British jurisdiction. He was not on the lawless streets of a war zone where he might have been caught in a crossfire. He was in custody, effectively on British soil, where he should have been safe from arbitrary violence.

The failure of the army to recognise the difference between a casualty of war and a murder victim is both sad and worrying. Britain aspires to a leading role in policing the world's trouble spots. It claims, as a liberal democracy, to have some moral authority over despotic regimes and some right to intervene in their affairs. That moral authority has been badly damaged by the general collapse in security for ordinary Iraqis. It is shredded when the army tolerates cold-blooded murder among its ranks.

Any reasonable observer could see that a civilian prisoner in custody deserved some legal protection. Now the courts have confirmed that view. An innocent man was killed while under British jurisdiction. A military investigation was botched. That must not be the end of it. There must be a new, civil investigation. Justice must be done for Baha Musa.